FYI, from Flywheel:
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This File: wp-content/plugins/download-manager/libs/class.FileSystem.php
This Function: imageThumbnail
This line $path = str_replace(site_url(‘/’), $abspath, $path); This is giving us the wrong path to the file.
Some plugins and themes make the assumption that one of these files/folders (most commonly wp-load.php) will be in the site root, and will try unsuccessfully to hardcode a link to it there. In order to fix it the plugin shouldn’t rely on ABSPATH when gathering URLs. As a temporary fix we set $abspath to the correct path by hand in wp-content/plugins/download-manager/libs/class.FileSystem.php.